util-lib: improve container detection logic

Previously, systemd-detect-virt was unable to detect "systemd-nspawn -a"
container environments, i.e. where PID 1 is a stub process running in host
context, as in that case /proc/1/environ was inherited from the host. Let's
improve that, and add an additional check for container environments where
/proc/1/environ is not cleaned up and does not contain the $container
environment variable:

The /proc/1/sched file shows the host PID in the first line. if this is not
1, we know we are running in a PID namespace (but not which implementation).

With these changes we should be able to detect container environments that
don't set $container at all.

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